1. Field of the Invention
The present invention relates to a thermal transfer printer, and more particularly, to a thermal transfer printer, which can reduce running costs of an ink ribbon.
2. Description of the Related Art
A thermal transfer printer 31 according to a related art will be described with reference to an invention disclosed in JP-A-2002-144616. As shown in FIG. 4, the thermal transfer printer 31 includes a printer main body 31a, a cassette mounting part 31b that can mount a ribbon cassette 39, to be described below, in the printer main body 31a. 
In addition, a platen roller 32 is disposed in the printer main body 31a, and a thermal head 33 is disposed above the platen roller 32.
Further, a recording paper 34 is fed and conveyed between the thermal heat 33 and the platen roller 32.
The thermal head 33 is supported by a head support table 35, and the head support table 35 is mounted to a head lever 36. The head lever 36 is pivotally supported on the support shaft 37. As the head lever 36 pivots, the thermal head 33 can come in contact with and be separated from (head up/down) the platen roller 32.
In a head up state of the thermal head 33, as the ribbon cassette 39 to be described below is mounted in the cassette mounting part 31b, an ink ribbon 38 is positioned between the platen roller 32 and the thermal head 33.
The ink ribbon 38 is accommodated in the ribbon cassette 39 in a state in which both ends thereof are wound around a supply reel 39a and a take-up reel 39b. 
As shown in FIG. 5, the ink ribbon 38 repeatedly forms ink surfaces 38a, 38b and 38c made of, for example, a yellow ink (Y), a cyan ink (C) and a magenta ink (M) and an overcoat layer 38d made of a transparent ink. These ink surfaces 38a, 38b, 38c and each of the overcoat layers 38d are formed in a size slightly larger than the printed area of the recording paper 34.
Two first markers 38b are formed in a transparent portion between the ink surfaces 38a of cyan and the ink surfaces 38a of yellow, and second markers 38c are respectively formed between the ink surfaces 38a of yellow and the ink surfaces 38a of magenta, and between the ink surfaces 38a of magenta and the ink surfaces 38a of cyan.
When detection of a ribbon detecting sensor 42 by the first markers 38b to be described below, a leading of the ink surface 38a of yellow is initially performed.
Further, a leading of the ink surface 38a of magenta and cyan is performed by the second marker 38c.
A transparent space part 38d with a dimension B is formed from the first and second markers 38b and 38c to the ink surfaces 38a of the respective colors.
A relief groove 39d is formed on the left side (in the drawing) of a partition wail 39e in the ribbon cassette 39, and a take-up opening 39f is formed on the bottom of the relief groove 39d (in an upper portion of the drawing). At a side plate 39c around the take-up opening 39f a guide roller 39 g made of a metal rod is rotatably supported.
During printing, the ink ribbon 38, which is closely in contact with the recording paper 34, is separated from the thermal head 33 at a predetermined separation angle, then passes by a guide roller 39g and is wound around the take-up reel 39b. 
A paper feed roller 40 and a pressure-contact roller 41 which is pressure-contacted to the paper feed roller 40 are disposed on the left side of the platen roller 32 in the drawing. While being pinched between the paper feed roller 40 and the pressure-contact roller 41, the recording paper 34 is conveyed in a direction indicated by an arrow A.
When the ribbon cassette 39 is mounted in the cassette mounting part 31b, the pressure-contact roller 41 is located in the relief groove 39d. 
In a portion where the relief groove 39d is located, an optical ribbon detecting sensor 42 is disposed. A reflective part (not shown) having glossiness which reflects a light irradiated from the ribbon detecting sensor 42 is formed in a partition wall 39e which faces the ribbon detecting sensor 42. A dimension from a heating element of the thermal head 33 in a head down state to the ribbon detecting sensor 42 is equal to a dimension B of the space part 38d of the ink ribbon 38.
A printing operation of the thermal transfer printer 31 according to the related art will be described. In the head up state of the thermal head 33, the ink ribbon 38 is wound by rotatingly driving the take-up reel 39b, and then when the ribbon detecting sensor 42 detects the first marker 38b, the winding of the ribbon stops. Then, a front end of the initial ink surface 38a of yellow of the ink ribbon 38 is arranged in a portion where the heating element of the thermal head 33 is formed, whereby a leading of the ink ribbon is performed.
While a front end 34a of the fed recording paper 34 is pinched between the paper feed roller 40 and the pressure-contact roller 41, a leading of the recording paper 34 is performed. Afterwards, after changing the state of the thermal head 33 to the head down state, the recording paper 34 is conveyed in the direction indicated by the arrow A by rotating the paper feed roller 40. At the same time, by selectively heating the heating element of the thermal head 33, an initial ink of yellow is thermally transferred so that an image of yellow is printed on the recording paper 34.
Then, after changing the state of the thermal head 33 to the head up state, the recording paper 34 is fed backward, and then a paper cue is performed and the ribbon detecting sensor 42 detects the second markers 38c of the ink surface 38a of magenta with the take-up reel 39b, so that a leading of the ink surface 38a of magenta is performed.
Then, after changing the state of the thermal head 33 to the head down state, an image of magenta is overlaid on the image of yellow. Afterwards, by repeating the above operation, an image of cyan is overlaid on the image of magenta so as to be printed, whereby it is possible to print an image of a desired color on the recording paper 34.
However, in the thermal transfer printer 31 according to the related art, since the ribbon detecting sensor 42 is disposed in the relief groove 39d of the ribbon cassette 39 which is estranged from the heating element of the thermal head 33, the dimension B of the space part 38d of the ink ribbon 38 corresponding to the dimension from the heating element of the thermal head 33 to the ribbon detecting sensor 42 is made long.
For this reason, portions of the ink ribbon 38 which do not contribute to printing increase, thus increasing running cost. Therefore, printing cost for one recording paper 34 becomes high.